Zinc/air cells are typically in the form of a miniature button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 12 mm and a height between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO.sub.2 alkaline cells and even larger sizes.
The miniature zinc/air button cell typically comprises an anode casing (anode cup), and a cathode casing (cathode cup). The anode casing and cathode casing each typically have a cup shaped body with integral closed end and opposing open end. After the necessary materials are inserted into the anode and cathode casings, the open end of the anode casing is typically inserted into the open end of the cathode casing and the cell sealed by crimping. The anode casing can be filled with a mixture comprising particulate zinc. Typically, the zinc mixture contains mercury and a gelling agent and becomes gelled when electrolyte is added to the mixture. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used. The cathode casing contains an air diffuser (air filter) which lines the inside surface of the cathode casing's closed end. The air diffuser can be selected from a variety of air permeable materials including paper and porous polymeric material. The air diffuser is placed adjacent air holes in the surface of the closed end of the cathode casing. Catalytic material typically comprising a mixture of particulate manganese dioxide, carbon and hydrophobic binder can be inserted into the cathode casing over the air diffuser on the side of the air diffuser not contacting the air holes. An ion permeable separator is typically applied over the catalytic material so that it faces the open end of the cathode casing.
The cathode casing can typically be of nickel plated stainless steel, for example, with the nickel plate forming the cathode casing's outside surface and stainless steel forming the casing's inside surface. The anode casing can also be of nickel plated stainless steel, typically with the nickel plate forming the casing's outside surface. The anode casing can be of a triclad material composed of stainless steel having an outer layer of nickel and an inner layer of copper. In such embodiment the nickel layer typically forms the anode casing's outside surface and the copper layer forms the anode casing's inside surface. The copper inside layer is desirable in that it provides a highly conductive pathway between the zinc particles and the cell's negative terminal at the closed end of the anode casing. An insulator ring of a durable, polymeric material can be inserted over the outside surface of the anode casing. The insulator ring is typically of high density polyethylene, polypropylene or nylon which resists flow (cold flow) when squeezed.
After the anode casing is filled with the zinc mixture and after the air diffuser, catalyst, and ion permeable separator is placed into the cathode casing, the open end of the anode casing can be inserted into the open end of the cathode casing. The peripheral edge of the cathode casing can then be crimped over the peripheral edge of the anode casing to form a tightly sealed cell. The insulator ring around the anode casing prevents electrical contact between the anode and cathode cups. A removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell. A portion of the closed end of the anode casing can function as the cell's negative terminal and a portion of the closed end of the cathode casing can function as the cell's positive terminal.
Typically, mercury is added in amount of at least one percent by weight, for example, about 3 percent by weight of the zinc in the anode mix. The mercury is added to the anode mix to improve interparticle contact between zinc particles in the anode mixture. This in turn improves electrical conductivity within the anode and thus results in increased cell performance, for example, higher actual specific capacity of the zinc (Amp-hour/g). Also addition of mercury tends to reduce the hydrogen gassing which can occur in the zinc/air cell during discharge and when the cell is placed in storage before or after discharge. The gassing, if excessive, increases the chance of electrolyte leakage, which can damage or destroy the hearing aid or other electronic component being powered. Many regions around the world now greatly restrict the use of mercury in electrochemical cells because of environmental concerns. If other potential gassing inhibitors are added to the anode mix instead of mercury, they either significantly reduce the anode conductivity or have to be added in quantity, thereby significantly reducing the cell's capacity (mAmp-hrs).
U.S. Pat. No. 3,897,265 discloses a representative zinc/air button cell construction with an anode casing inserted into the cathode casing. There is disclosed an insulator between the anode and cathode casings. The anode comprises zinc amalgamated with mercury. The cell includes an assembly comprising an air diffuser, cathode catalyst, and separator at the closed end of the cathode casing facing air holes in the surface of the cathode casing.
U.S. Pat. No. 5,279,905 discloses a miniature zinc/air cell wherein little or no mercury has been added to the anode mix. Instead, the inner layer of the anode casing has been coated with a layer of indium. The disclosed anode casing can be a triclad material composed of stainless steel plated on the outside surface with nickel and on the inside surface with copper. The copper layer is at least 1 microinch (25.4.times.10.sup.-6 mm). The reference discloses coating the copper layer on the anode casing's inside surface with a layer of indium. The indium layer is disclosed as being between about 1 microinch and 5 microinches (25.4.times.10.sup.-6 mm and 127.times.10.sup.-6 mm).
It is desired to improve the interparticle conductivity of the zinc in the anode of a zinc/air cell without adding mercury and without significantly increasing gassing.
It is desired to improve the actual specific capacity (m-Amp/g) of the zinc in the anode without adding mercury.